“I had designed a few other projects using Raspberry Pi, so I had extra Raspberry Pi’s laying around,” Barry explains. “[I] decided to put my extra Raspberry Pi 3B+ to good use as the +5 V power source [of the] automatic timer for the phone ring killer circuit.”
In the lab
We feel like everyone needs some kind of maker room if they can manage it. Barry though is a professional.
“I have my own electronics lab in my house,” Barry mentions. “Including lots of spare parts, and very sophisticated test equipment, having worked as an independent consultant engineer for many years. That makes building these tiny projects a breeze for me.”
The tiny project involves a little more hardware than software, as it is connected directly into the phone. You can follow along to Figure 1 for the explanation by Barry on how it works.
“The telephone line attaches to the Ring Detector (components C1, D1, D2, D3, R1, and the input to IC1 the optocoupler). Zener diodes D1 and D2, combined with capacitor C1, allow only high-voltage AC signals to reach the input of the optocoupler. Normal voice and dialling tones don’t affect the ring detector at all. The Ring Signal satisfies these high-voltage AC requirements as it is an 80 V RMS (113 V zero to peak) AC signal superimposed on the 47 V DC idle phone line voltage.
“When the phone rings, the input of the optocoupler gets activated. The output of the optocoupler then turns on driving R2 to nearly 5 V DC into the gate of the MOSFET Q1, which momentarily loads down the telephone line with 680 ohm resistor R3 signalling the phone company to shut off the ring signal. This happens so fast that you don’t even hear the phone ring. This describes the operation of the circuit when it is in the ‘Sleep’ mode.”
The Raspberry Pi controls switching sleep mode on or off via powering GPIO pins with a Python program that gets the time from NTP (Network Time Protocol). There’s also a manual toggle in case you’re going to bed early.
Like a baby
“I get a much better night’s sleep just knowing that I will not have my slumber interrupted by another annoying telemarketing call at six in the morning,” Barry tells us.
When we asked about the success of the project, Barry seems not to think there’s much mass appeal for a project like this, as everyone just uses ‘do not disturb’ on their smartphones. While perhaps true, we still think there are plenty of folks who would love to have some manual control over their old landline.
